1. Field
Embodiments of the invention relate to programmable digital devices. More specifically, one embodiment of the invention relates to a system, apparatus and method for detecting and preventing tampering with programmable digital devices.
2. General Background
Analog communication systems are rapidly giving way to their digital counterparts. High-definition television (HDTV) broadcasts have already begun in most major cities on a limited basis, with the goal for all programming to be HDTV broadcasts. Similarly, the explosive growth of the Internet and the World Wide Web have resulted in a correlative growth in the increase of downloadable audio-visual files, such as MP3-formatted audio files, as well as other content.
Simultaneously with, and in part due to this rapid movement toward digital communications, there have been significant advances in digital recording devices. Digital versatile disk (DVD) recorders, digital VHS video cassette recorders (D-VHS VCR), CD-ROM recorders (e.g., CD-R and CD-RW), MP3 recording devices, and hard disk-based recording units are but merely representative of the digital recording devices that are capable of producing high quality recordings, without the generational degradation (i.e., increased degradation between successive copies) known in the analog counterparts.
As a result, due to fears of unauthorized and uncontrolled copying such digital content, content providers such as the motion picture and music industries have become reluctant in providing downloadable digital content. In fact, there are requests for copy protection initiatives that extend beyond the traditional role of conditional access (CA), namely scrambling and descrambling of content for real-time viewing and/or listening.
One initiative in development is the implementation of CA technology in a subscriber terminal device (e.g., set-top box) using a secure embedded processor. This secure embedded processor would be configured to store sensitive data, namely cryptographic keys, certificates, microcode, gate configuration data or other persistent information for example, within an internal one-time programmable (OTP) memory.
Traditionally, OTP memory may be implemented with one or more fuses or anti-fuses. A “fuse” (or an emulation of such) involves the destructive removal of an internal interconnection to permanently change a bit in OTP memory from a manufactured default state of logic “1” (ONE) to logic “0” (ZERO). Once performed, this transition can never be reversed. An “anti-fuse,” however, has all bits programmed to ZERO by default and the transition is to ONE. Both of these configurations enable the OTP memory to permanently disable external read or write accesses after the sensitive data has been loaded. However, by their nature, OTP memory is susceptible to security attacks.
For instance, security attacks may be conducted to gain unauthorized access to read the sensitive data in order to clone or create methods to circumvent it. Such attacks may be conducted to alter or substitute data in place of the bona fide preloaded, sensitive data to circumvent normal operation of the secure processor. As a result, the operations of the secure processor may be disrupted or incoming content made be decrypted by unauthorized parties.
One of the simpler methods of attack is to “blind write” over existing key data to change the OTP keys to a deterministic (known) value. One such method is to take advantage of the fuse or anti-fuse logic by changing the unique keys from their original value to all ONEs or ZEROes, depending upon the fuse technology deployed. More specifically, if all ones (or conversely all ZEROes) are written into the memory location reserved for the OTP keys, the existing contents will be transitioned to a deterministic value, regardless of the prior state, even without being able to read the memory.
With known values now installed in the OTP memory, content can be then freely accessed through the transmission of an entitlement management message (EMM) entitling the device to decode all content. Alternatively, the subscriber terminal device may be permanently rendered non-functional if a malicious blind write is made to a memory area of programmable logic containing gate configuration data, or a memory area of an embedded microcontroller or digital signal processor containing microcode or algorithms.